The invention relates to a wheel slippage control system, more particularly, to an anti-lock brake control system and/or a drive-slip control system. The system includes wheel speed sensors which generate signals corresponding to the wheel speed. An evaluation circuit receives the wheel speed signals and generates a reference speed and slippage signals for brake pressure control, the curve of the reference speed being approximated to the curve of the vehicle speed. First brake pressure control devices actuated by the slippage signals cause a variation in brake pressure which depends on wheel slippage; the devices permit pressure build-up, pressure reduction, and maintaining the pressure constant.
It is a pronounced object of all known wheel control algorithms to maintain the directly controlled wheel slippage within a smallest possible range around the maximum of the slippage curve. A typical plot of braking force versus wheel slippage is shown in FIG. 5. The Bosch system ABS2 is based on the principle of increasing the wheel slippage until the wheel-tire-system becomes unstable. Subsequently, the wheel slippage is decreased until the wheel-tire-system has reached again a stable point on the slippage curve. When this point is reached, the slippage is increased again and the control cycle is repeated. Instability detection is a main feature of the anti-lock brake control system.
When the slippage curve decreases abruptly after the maximum, the wheel rapidly decelerates when the maximum is surpassed even when the braking torque is maintained only slightly above the value which corresponds to the maximum of the slippage curve. In this case, instability can be definitely detected without problems. It is possible, however, that the rapid wheel deceleration causes a great wheel slippage and, hence, a substantial loss of braking power and lateral guidance.
When, as opposed thereto, the slippage curve hardly decreases after the maximum, as on ice, for example, the wheel speed decreases only slowly when the braking torque is maintained just above the value which corresponds to the maximum of the slippage curve. In this case instability detection is much more difficult. It is possible that instability might be detected only at a great wheel slippage which in turn may cause a high loss in lateral guidance.
In order to achieve a good braking effect and a highest possible lateral guidance, the slippage curve maximum must not be exceeded. In order to meet further demands regarding lateral guidance, the tire slippage should be kept below .lambda.*, where .lambda.* signifies the slippage where the maximum of the slippage curve is reached. In FIG. 5, e.g., the maximum braking force occurs when .lambda.*=20%. Since the wheel-tire-system is stable for slippage values smaller than .lambda.*, instability detection cannot be used to control for slippage values smaller than .lambda.*.
Controlling for slippage values smaller than .lambda.* would be possible if the slippage curve and the instantaneous slippage were known. Since a determination of this kind requires auxiliary signals which are difficult to measure, like the vehicle speed, an alternative must be found to accomplish the object more economically. One such possible alternative is increasing the slippage curve.
Generally, the slope of the slippage curve decreases monotonously with a rising slippage. The relation between wheel slippage and slope of the slippage curve is then clear. The requirement of controlling for a certain slippage value can then be replaced by the requirement of controlling for a certain slope of the slippage curve. The greater the slope of the slippage curve at the operating point, the more stable is the operating point. The smaller the slope of the slippage curve, the smaller is the stability of the operating point. A negative slope means a instable operating point. The slope is hence a measure for the stability reserve of the operating point.
Earlier applications already suggested estimating the slope of the slippage curve from the bounce response of the wheel. For this purpose, the brake pressure is abruptly increased and the wheel speed is subsequently analyzed. This method involves two problems:
1. In case of smaller friction coefficients, the inaccuracy of the wheel speed signal makes an analysis of the wheel speed more difficult. On ice, for example, .lambda.*.apprxeq.4% and the pressure bounces, which can only be very small, cause only very small slippage changes which cannot be reliably analyzed because of the aforesaid inaccuracy. PA1 2. The wheel behavior after the pressure bounce is affected by several interfering factors such as interferences in the friction coefficient of the road and deviations of the forces regarding the contact surface of the tire. In case of smaller pressure bounces, these changes can completely override the transient wheel behavior because of the pressure bounce. This renders a reliable analysis of the wheel speed with respect to the slope of the slippage curve impossible. An increase of the pressure bounces meant an improvement, however, the deviations from the desired slippage would increase, causing disadvantageous losses in braking power, lateral guidance and comfort.